Sediment Monitoring Bias by Autosampler in Comparison with Whole Volume Sampling for Parking Lot Runoff

A field study was conducted to assess biases of suspended sediment concentration (SSC) analyses (ASTM standard D 3977–97) performed on discrete samples obtained by auto-sampler in comparison with actual sediment concentrations from whole volume sampling. Research results indicate that the biases attributed to monitoring of sediment event mean concentration (EMC) and particle size distribution (PSD) in parking lot runoff by automated samplers (non-isokinetic) can be insignificant. Large volume samples (~15,000 L) of the full pipe cross-section were taken from a stormwater sewer system for eighteen storm events over two years to monitor for EMCs and PSDs. Concurrently, flow weighted grab samples were obtained by automatic samplers throughout the entire large volume sampling period. Thus characteristics of sediments were compared from a "whole storm" sample with that of "sub-samples" obtained by an autosampler using non-isokinetic sampling. SSCs and particle size distributions were compared for the two respective field sampling methods. The two methods showed a strong correlation for median sediment EMCs (R² =.9801, n=18). Biases to particle size distributions were found to be largely confined to the large particles (>160 um) representing less than 25% of the load by mass. Moreover, sediment particles captured by the whole volume sampling and autosampler were not significantly different (α=0.05) for D₅₀'s=0.067= and 0.047 microns respectively (particle size at which 50% of the sample mass is less).